578 research outputs found
Order-disorder phase change in embedded Si nano-particles
We investigated the relative stability of the amorphous vs crystalline
nanoparticles of size ranging between 0.8 and 1.8 nm. We found that, at
variance from bulk systems, at low T small nanoparticles are amorphous and they
undergo to an amorphous-to-crystalline phase transition at high T. On the
contrary, large nanoparticles recover the bulk-like behavior: crystalline at
low T and amorphous at high T. We also investigated the structure of
crystalline nanoparticles, providing evidence that they are formed by an
ordered core surrounded by a disordered periphery. Furthermore, we also provide
evidence that the details of the structure of the crystalline core depend on
the size of the nanoparticleComment: 8 pages, 5 figure
Phase ordering of two-dimensional symmetric binary fluids: a droplet scaling state
The late-stage phase ordering, in dimensions, of symmetric fluid
mixtures violates dynamical scaling. We show however that, even at 50/50 volume
fractions, if an asymmetric droplet morphology is initially present then this
sustains itself, throughout the viscous hydrodynamic regime, by a
`coalescence-induced coalescence' mechanism. Scaling is recovered (with length
scale , as in ). The crossover to the inertial hydrodynamic
regime is delayed even longer than in ; on entering it, full symmetry is
finally restored and we find , regardless of the initial state.Comment: 4 pages, three figures include
Incomplete equilibrium in long-range interacting systems
We use a Hamiltonian dynamics to discuss the statistical mechanics of
long-lasting quasi-stationary states particularly relevant for long-range
interacting systems. Despite the presence of an anomalous single-particle
velocity distribution, we find that the Central Limit Theorem implies the
Boltzmann expression in Gibbs' -space. We identify the nonequilibrium
sub-manifold of -space characterizing the anomalous behavior and show
that by restricting the Boltzmann-Gibbs approach to this sub-manifold we obtain
the statistical mechanics of the quasi-stationary states.Comment: Title changed, throughout revision of the tex
Unzipping Vortices in Type-II Superconductors
The unzipping of vortex lines using magnetic-force microscopy from extended
defects is studied theoretically. We study both the unzipping isolated vortex
from common defects, such as columnar pins and twin-planes, and the unzipping
of a vortex from a plane in the presence of other vortices. We show, using
analytic and numerical methods, that the universal properties of the unzipping
transition of a single vortex depend only on the dimensionality of the defect
in the presence and absence of disorder. For the unzipping of a vortex from a
plane populated with many vortices is shown to be very sensitive to the
properties of the vortices in the two-dimensional plane. In particular such
unzipping experiments can be used to measure the ``Luttinger liquid parameter''
of the vortices in the plane. In addition we suggest a method for measuring the
line tension of the vortex directly using the experiments.Comment: 19 pages 15 figure
Rheology of distorted nematic liquid crystals
We use lattice Boltzmann simulations of the Beris--Edwards formulation of
nematodynamics to probe the response of a nematic liquid crystal with
conflicting anchoring at the boundaries under shear and Poiseuille flow. The
geometry we focus on is that of the hybrid aligned nematic (HAN) cell, common
in devices. In the nematic phase, backflow effects resulting from the elastic
distortion in the director field render the velocity profile strongly
non-Newtonian and asymmetric. As the transition to the isotropic phase is
approached, these effects become progressively weaker. If the fluid is heated
just above the transition point, however, another asymmetry appears, in the
dynamics of shear band formation.Comment: 7 pages, 4 figures. Accepted for publication in Europhys. Let
RNA denaturation: excluded volume, pseudoknots and transition scenarios
A lattice model of RNA denaturation which fully accounts for the excluded
volume effects among nucleotides is proposed. A numerical study shows that
interactions forming pseudoknots must be included in order to get a sharp
continuous transition. Otherwise a smooth crossover occurs from the swollen
linear polymer behavior to highly ramified, almost compact conformations with
secondary structures. In the latter scenario, which is appropriate when these
structures are much more stable than pseudoknot links, probability
distributions for the lengths of both loops and main branches obey scaling with
nonclassical exponents.Comment: 4 pages 3 figure
Free Energy Self-Averaging in Protein-Sized Random Heteropolymers
Current theories of heteropolymers are inherently macrpscopic, but are
applied to folding proteins which are only mesoscopic. In these theories, one
computes the averaged free energy over sequences, always assuming that it is
self-averaging -- a property well-established only if a system with quenched
disorder is macroscopic. By enumerating the states and energies of compact 18,
27, and 36mers on a simplified lattice model with an ensemble of random
sequences, we test the validity of the self-averaging approximation. We find
that fluctuations in the free energy between sequences are weak, and that
self-averaging is a valid approximation at the length scale of real proteins.
These results validate certain sequence design methods which can exponentially
speed up computational design and greatly simplify experimental realizations.Comment: 4 pages, 3 figure
New Trends in the Quality Control of Enantiomeric Drugs: Quality by Design-Compliant Development of Chiral Capillary Electrophoresis Methods
Capillary electrophoresis (CE) is a potent method for analyzing chiral substances and is commonly used in the enantioseparation and chiral purity control of pharmaceuticals from different matrices. The adoption of Quality by Design (QbD) concepts in analytical method development, optimization and validation is a widespread trend observed in various analytical approaches including chiral CE. The application of Analytical QbD (AQbD) leads to the development of analytical methods based on sound science combined with risk management, and to a well understood process clarifying the influence of method parameters on the analytical output. The Design of Experiments (DoE) method employing chemometric tools is an essential part of QbD-based method development, allowing for the simultaneous evaluation of experimental parameters as well as their interaction. In 2022 the International Council for Harmonization (ICH) released two draft guidelines (ICH Q14 and ICH Q2(R2)) that are intended to encourage more robust analytical procedures. The ICH Q14 guideline intends to harmonize the scientific approaches for analytical procedures’ development, while the Q2(R2) document covers the validation principles for the use of analytical procedures including the recent applications that require multivariate statistical analyses. The aim of this review is to provide an overview of the new prospects for chiral CE method development applied for the enantiomeric purity control of pharmaceuticals using AQbD principles. The review also provides an overview of recent research (2012–2022) on the applicability of CE methods in chiral drug impurity profiling
Scaling in DNA unzipping models: denaturated loops and end-segments as branches of a block copolymer network
For a model of DNA denaturation, exponents describing the distributions of
denaturated loops and unzipped end-segments are determined by exact enumeration
and by Monte Carlo simulations in two and three dimensions. The loop
distributions are consistent with first order thermal denaturation in both
cases. Results for end-segments show a coexistence of two distinct power laws
in the relative distributions, which is not foreseen by a recent approach in
which DNA is treated as a homogeneous network of linear polymer segments. This
unexpected feature, and the discrepancies with such an approach, are explained
in terms of a refined scaling picture in which a precise distinction is made
between network branches representing single stranded and effective double
stranded segments.Comment: 8 pages, 8 figure
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